The present invention relates to a method and arrangements in a telecommunication system. In particular it relates to a method and arrangements for Multi User Multiple-Input-Multiple-Output signaling in the telecommunication system.
In the cellular telecommunication system which typically comprises, as illustrated in FIG. 1, a core network 1, a radio access network 2, User Equipments (UEs) 4 and base stations 3 multiple transmit antennas can be used for achieving high data rates in various ways. A multiple-input-multiple-output (MIMO) channel is formed if the receiver also has multiple antennas. One application in such a setup is to strive for high peak rates to a single user. By transmitting on several layers, i.e., wherein the information is transmitted on several bit streams the information is spread in the spatial domain, and substantial improvement in data rate can be achieved under favorable channel conditions. This is called single user MIMO (SU-MIMO) since the data on several layers is intended for a single receiver/user/UE/terminal. FIG. 2 shows an example of a base station 20, with multiple transmit antennas 23, that is transmitting in SU-MIMO mode to a single UE 21. As shown in FIG. 2 several layers 22 are transmitted to a single UE 21. In FIG. 2 the UE is also transmitting to the base station 20 using several layers. The telecommunication system may be an LTE-system which is an evolution of the UMTS.
The number of simultaneously transmitted layers depends highly on the properties of the MIMO channel. Because of, for example fading, usually the MIMO channel does not support more than one layer transmission to a single UE. This limits the data rate and means that spatial multiplexing gain is not possible. To still reach high system capacity it might be beneficial to transmit only a limited number of layers to a single user and instead schedule several users on the same physical resource (e.g. time-frequency-code tile) and use the spatial domain (layers) to separate the users. In essence, layers belonging to different users are transmitted on the same physical resource. Even if the channel to a particular user is such that it does not support multiple layers, which means that it is not possible to transmit multiple layers to that particular user, spatial multiplexing gain on a system level can be achieved as long as the user can efficiently suppress the layers transmitted to the other users. This technique is sometimes referred to as multi-user MIMO (MU-MIMO) and is especially attractive in high load scenarios with many active users as described in 3GPP R1-063130, “System level comparison between MU- and SU-MIMO for downlink precoding systems with four transmit antennas”, Ericsson. TSG-RAN WG I #47, November 2006. FIG. 3 shows an example of a base station 20, with multiple transmit antennas 23, that is transmitting in MU-MIMO mode to multiple UEs 31, 32 and 33. As shown in FIG. 3, different layer 34, 35 and 36 is transmitted to each UE 31, 32 and 33. As illustrated in FIG. 3, each UE is also transmitting to the base station 20 using different layers.
In the Long Term Evolution (LTE) standardization process, there is agreement on the support of SU-MIMO and MU-MIMO in the downlink and that there will be the possibility to semi-statically switch between these two modes. Each UE (user/receiver) can in the MU-MIMO mode receive zero or one layer. There are basically three proposals for MU-MIMO support under discussion:                1. Classical space-division multiple access (SDMA) for correlated antenna array setups described in 3GPP R1-072464, “MU-MIMO for E-UTRA DL”, Ericsson, TSG-RAN WG1 #49, May 2007.        2. Zero-forcing beamforming described in 3GPP R1-071510, “Details of Zero-Forcing MU-MIMO for DL E-UTRA”, Freescale Semiconductor Inc., TSG RAN WG 1 #48bis, March 2007.        3. Per user unitary rate control (PU2RC) described in 3GPP R1-060335, “Downlink MIMO for EUTRA”, Samsung, TSG RAN WG1 #44, February 2006.        
It has not yet been decided which scheme to support and many remaining details of the standardization of MU-MIMO are hence lacking. One problem that occurs regardless of MU-MIMO mode is how the UE is to know the power offset between a power reference, for instance reference symbols (RS), and data symbols transmitted to the UE. The base station has a certain transmit power of which a certain amount is used to transmit data symbols to a particular UE. The power offset indicates how much power is used to transmit the data symbols in relation to the power reference. This power offset is needed in order to support efficient demodulation in the UE when higher order modulation alphabets like 16 QAM or other higher order modulation schemes are used. The power offset can vary dynamically because of different power settings at the base station (also referred to as Node B or eNode B). In case of MU-MIMO, the power offset can however also fluctuate because of a varying number of multiplexed UEs on the same physical resource. The available transmission power may for example be equally divided among UEs scheduled on different layers in the MU-MIMO mode, meaning less power per UE when several UEs are multiplexed.